In a typical telephone switching system, a line interface circuit is used to provide coupling to a subscriber telephone line served by the system. The line interface circuit usually comprises a transformer having a primary winding being split into equal halves for connection to tip and ring leads of the subscriber line and a secondary winding for connection to a hybrid circuit. Energizing direct current is supplied via tip and ring feed resistors and the split primary winding for operation of a telephone instrument or the like, connected at the far end of the telephone line. The line interface circuit may also include a tertiary winding, sometimes referred to as a balance winding, similar to that disclosed by V.V. Korsky in U.S. Pat. No. 4,103,112 issued on July 25, 1978. In any case the halves of the split primary winding are connected in series by a capacitance device, usually referred to as a capacitor which conducts a.c. signal currents. If it were not for the capacitor, the a.c. signal currents would otherwise have to traverse some other path, in this example the tip and ring feed resistors. In fact, some a.c. signal currents at the lower end of the voice band do traverse the tip and ring feed resistors because the conductance of the capacitor at these frequencies tends toward the conductances of the feed resistors. This is of deleterious effect to telephone line interface operation which is the subject of my U.S. Pat. No. 4,864,608 issued 5 Sept. 1989, also in the name of M.S. Moisin. The capacitance value of the capacitor is a critical factor in defining the terminating impedance of the line interface circuit. Therefore within an operating territory of any telephone company this capacitance value for any particular family of line interface circuit types is predetermined.
Recently, in some operating telephone companies, a telemetry service has been introduced whereby a metered measure, of a supplied commodity, is communicated from a subscriber's premise, to the supplier of the commodity via the telephone system. Such commodity may be water, gas, or electricity for example. In one such arrangement, telemetry equipment at the subscriber's premise is poled in response to a reversal of the polarity of the energizing current being applied at the line interface circuit This is simply accomplished by switching the tip and ring leads between the ends of the split primary winding of the transformer for a few seconds during which time it is intended that telemetry equipment has responded. Thereafter, the normal connection arrangement is restored.
Unfortunately, in many such arrangements, the accumulated charge, in the capacitor connected between the halves of the split primary winding, causes a current surge along the telephone line sufficient to induce a momentary operation of the ringer in the subscriber's telephone instrument. As telemetry is conveniently performed while the subscriber's station set is in an idle condition, it is therefore usually performed sometime during the first few hours following midnight, that is when telephones are least likely to be in use.
Heads are a nodding, PA1 Sleep is a falling, PA1 The witching hour draws neigh, PA1 When a telephone tinkle, PA1 From telemetric wrinkle, PA1 Raises a red eyed insomniac's sigh. PA1 Night after night, PA1 Insomniac's plight, PA1 Is repeated with frustrated cry; PA1 Till with telephone gripped, PA1 Revenge's cup sipped, PA1 The instrument's hurled far and high.
Those telephone subscribers who are light sleepers, but nevertheless want to have a telephone nearby, can be disturbed by the initial tinkle of battery reversal, which is then followed a second or two later by a second tinkle, when the normal connection at the line interface circuit is restored.